(1) Field of the Invention
The present invention relates to a PN code demodulator and a code-division multiplex communication apparatus using the PN code demodulator, and more particularly, to a PN code demodulator for demodulating a baseband signal modulated using a PN code, and a code-division multiplex communication apparatus including a transmitting section for transmitting data signals modulated by means of a PN code via a plurality of channels and a receiving section for demodulating signals from the respective channels by means of the PN code demodulator to thereby restore the data signals.
(2) Description of the Related Art
A communication apparatus designed to carry out spread spectrum (SS) modulation by using a PN code (pseudonoise code) is known as a communication apparatus that permits multiplexing and is tolerant of interference. Thus, this type of communication apparatus is used for satellite communications dealing with feeble radio waves or in an environment wherein the strength of output radio waves is regulated.
First, the principles of spread spectrum communications and a conventional PN code demodulator for achieving the spread spectrum communications are explained.
FIG. 12 schematically illustrates a conventional spread spectrum communication system in which no multiplexing is carried out. At a transmitting side, data is modulated with a PN code by a PN code modulator 101, is further digitally modulated with a carrier wave by a binary phase-shift keying modulator (PSKMOD) 102, and then transmitted. On a receiving side, the received signal is digitally demodulated by a synchronous detector 103 and is further subjected to PN-code demodulation by a PN code demodulator 104, and the extracted information is output.
The PN code modulator 101 is composed of an exclusive-OR (EX-OR) circuit 101a and a PN code generator 101b. One bit of information and one series (one frame) of PN code associated therewith are subjected to exclusive-OR operation, whereby the resulting data is spread.
As an example of such PN code, an M-series PN code having a code length of "7" will be explained. In the case where a 7-bit PN code "1110100" is associated with a 1-bit signal "1" of data and an exclusive-OR operation is performed on these two items of data, then output data (spread data) S from the PN code modulator 101 is "0001011", as shown in FIG. 13. Where the data contains a signal "0", the resulting spread data S is "1110100".
The binary phase-shift keying modulator 102 subjects the spread data to binary phase-shift keying modulation (BPSK), and then transmits the modulated signal. As shown in FIG. 14, the binary phase-shift keying modulator 102 outputs, for example, a 0-degree phase carrier wave (blank part in the figure) for the bit "1" of the spread data S, and outputs a 180-degree phase carrier wave (shaded part in the figure) for the bit "0" of the spread data, as a transmission wave having a fixed amplitude.
On the receiving side, as shown in FIG. 15, the synchronous detector 103 converts (demodulates) the spread data, which has been subjected to the binary phase-shift keying modulation by means of the carrier wave, into a baseband signal W "0001011".
The PN code demodulator 104 serves to extract the data from the baseband signal or the spread data. As shown in FIG. 12, the PN code demodulator 104 comprises shift registers 104a corresponding in number to the code length of the PN code, a PN code generator 104b for generating in parallel PN codes which are identical with those generated by the PN code generator 101b of the transmitting side, exclusive-OR (EX-OR) circuits 104c corresponding in number to the code length of the PN code, and a majority logic circuit 104d. The PN code generator 104b includes a ROM, a switch, etc., and the majority logic circuit 104d includes a voltage adder and a majority comparator.
With this arrangement, the spread data demodulated and converted into the baseband signal W is successively input to the shift registers 104a whose number corresponds to the code length. The outputs of the shift registers 104a are supplied to the corresponding ones of the exclusive-OR circuits 104c. The exclusive-OR circuits 104c are also supplied with PN codes which are identical with those generated at the transmitting side and which are supplied in parallel from the PN code generator 104b. When the baseband signal W corresponding to one frame has been input to the shift registers 104a, the majority logic circuit 104d adds up the output voltages of the exclusive-OR circuits 104c for majority comparison.
Specifically, as shown in FIG. 16, in the case where the shift registers 104a are supplied with spread data which has been demodulated and converted into a baseband signal W "0001011", for example, the exclusive-OR circuits 104c perform exclusive-OR operations on the baseband signal W "0001011" and the PN code "1110100" and provide outputs R "1111111". The majority logic circuit 104d adds up the output bits and compares the sum "7" with the criterion value "4" which is the least majority of the total of bits (PN code length) "7". In the illustrated case, the sum is greater than the criterion value, and accordingly, the majority logic circuit 104d outputs data "1". When the sum is smaller than the majority criterion value, data "0" is output.
Thus, in spread spectrum communications, even if noise is contained in the information in the course of transmission, the information sent from the transmitting side can be reliably restored at the receiving side. In this case, the longer is the PN code length, the higher is the reliability of restoration of the data becomes at the receiving side. Although in the illustrated example, the PN code has a code length of 7 bits, the PN code usually has a code length of several hundreds to several thousands of bits.
The following explains a conventional code-division multiplex communication apparatus wherein multiplex communication is achieved by the above-described spread spectrum communication technique.
Code-division multiplex communications utilize a feature of the spread spectrum communication technique, that is, the property that a PN code, which is identical with that used at the transmitting side for the spreading, can be used for the inverse spreading (correlation) at the receiving side to extract the original signal. Namely, in cases where different PN codes (i.e., low cross-correlation codes) are used for different channels, a number of items of data can be transmitted within an identical frequency band.
FIG. 17 illustrates the basic arrangement of a conventional transmitting apparatus used for code-division multiplex communications. As illustrated, n exclusive-OR (EX-OR) circuits 110 perform exclusive-OR operations on data items 1 to n from respective channels CH1 to CHn and PN codes 1 to n, and supply exclusive-OR outputs X1 to Xn to a majority logic circuit 111. The majority logic circuit 111 determines a majority of the values of the signals belonging to the same time slot, and supplies a modulator 112 with the signal value representing the majority of the exclusive-OR outputs X1 to Xn which are binary signals. The arrangement of the majority logic circuit 111 will be described later. It is here assumed that the number of the channels is an odd number (n: odd number).
The modulator 112 comprises a double balanced modulator (DBM) 112a and an amplifier 112b. The double balanced modulator 112a subjects a carrier wave to phase modulation by using the output of the majority logic circuit 111, and the amplifier 112b subjects the modulated signal to RF amplification for transmission.
The above transmitting apparatus will be now explained in more detail on the assumption that the PN code used is an M-series PN code having a maximum-length connection tap of [3, 1] and a code length of "7". Also, it is assumed that the number of channels is three. Namely, as shown in FIG. 18, PN codes 1, 2 and 3 are respectively "1110100", "0111010" and "0011101".
The PN codes 1 to 3 are supplied to three exclusive-OR circuits 110, respectively, as shown in FIG. 19, and the exclusive-OR circuits 110 are also supplied with data "0", "0" and "1", for example, from three channels CH1 to CH3. In each exclusive-OR circuit 110, one frame of the PN code is associated with one bit of the data transmitted from the corresponding channel.
Consequently, the data on the channel CH1 is converted to a signal X1 "1110100", the data on the channel CH2 is converted to a signal X2 "0111010", and the data on the channel CH3 is converted to a signal X3 "1100010".
The majority logic circuit 111 determines a majority of the signals X1 to X3 of the channels on a bit-by-bit basis. According to the majority determination, the value "1" is output when a majority of the binary signals supplied in parallel from the channels has the value "1", and the value "0" is output when a majority of the binary signals has the value "0". Specifically, as shown in FIG. 20, at timing b.sub.1, the signals X1 to X3 of the channels CH1 to CH3 are all "0", and thus the output Y of the majority logic circuit 111 is "0". At timing b.sub.2, the signal X1 of the channel CH1 is "0" while the signals X2 and X3 of the channels CH2 and CH3 are "1". Since a majority of the signals have the value "1", the output Y of the majority logic circuit 111 is "1". The PN code corresponding to one frame is subjected to majority operation in this manner, and the result "1110010" is obtained.
The majority logic circuit 111 can be constituted by a combination of AND gates and an OR gate, as shown in FIG. 21(A). FIG. 21(B) illustrates a logical expression employed in the majority logic circuit 111, and FIG. 21(C) illustrates a truth table of the circuit 111.
The output Y of the majority logic circuit 111 is subjected to binary phase-shift keying modulation by the double balanced modulator 112a, as shown in FIG. 22. In the amplitude characteristic shown in the figure, the blank part indicates a 0-degree phase carrier wave having a fixed amplitude, and the shaded part indicates a 180-degree phase carrier wave having the fixed amplitude. For better understanding of the correlation of codes of the transmitted wave, amplitude is converted such that the 0-degree phase and the 180-degree phase are shown on the plus (+) and minus (-) sides, respectively, of the vertical axis. The converted transmission wave W is expressed as "1, 1, 1, -1, -1, 1, -1".
A conventional receiving apparatus used for code-division multiplex communications will be now explained.
FIG. 23 illustrates the basic arrangement of a conventional receiving apparatus for code-division multiplex communications. In the figure, PN code generators 113a to 113c output PN codes 1 to 3 respectively identical with the PN codes 1 to 3 generated in the aforementioned transmitting apparatus. Double balanced modulators 115a to 115c previously subject an oscillation signal from a local oscillator 114 to phase modulation, by using the PN codes 1 to 3, respectively. Using the outputs of the double balanced modulators 115a to 115c, another set of double balanced modulators 116a to 116c acquires a correlation with the transmitted wave from the transmitting side for the individual channels. The data of each channel is passed through a band-pass filter 117a, 117b or 117c having a frequency band characteristic approximately twice the data rate, whereby each of PSK demodulators 118a to 118c is supplied with the result (integral) of the correlations over one frame of the PN code as an amplitude output. The PSK demodulators 118a to 118c demodulate data 1 to 3 by means of synchronous detection.
As shown in FIG. 24, the double balanced modulator 115a previously subjects the oscillation signal from the local oscillator 114 to phase modulation by using the PN code 1 "1110100", and outputs the converted signal PN1 "1, 1, 1, -1, 1, -1, -1". Similarly, the double balanced modulator 115b previously subjects the oscillation signal from the local oscillator 114 to phase modulation by using the PN code 2 "0111010", and outputs the converted signal PN2 "-1, 1, 1, 1, -1, 1, -1", as shown in FIG. 25. Further, as shown in FIG. 26, the double balanced modulator 115c previously subjects the oscillation signal from the local oscillator 114 to phase modulation by using the PN code 3 "0011101", and outputs the converted signal PN3 "-1, -1, 1, 1, 1, -1, 1".
Detecting the correlations at the double balanced modulators 116a to 116c and determining the data on the basis of the results of integration from the bandpass filters 117a to 117c is equivalent to acquiring the inner product of the waveform of the received wave W "1, 1, 1, -1, -1, 1, -1" and each of the converted signals PN1 to PN3.
Namely, for the channel CH1, ##EQU1## for the channel CH2, ##EQU2## and for the channel CH3, ##EQU3##
In this case, if the result of calculation has a positive sign (+), "0" is restored as data, and if the result of calculation has a negative sign (-), "1" is restored as data.
Thus, the data "0", "0" and "1" of the transmitting side can be reliably restored. In other words, multiplex communication is carried out by using the spread spectrum communication technique.
Using the majority logic circuit 111 in the transmitting apparatus leads to reduction in the quantity of hardware, as compared with an existing transmitting apparatus in which an ordinary adder circuit is arranged subsequently to the three exclusive-OR circuits 110, and an easy-to-adjust communication apparatus can be provided.
The conventional PN code demodulator 104 for a spread spectrum communication system, however, requires unit registers including the shift registers 104a and the exclusive-OR circuits 104c respectively corresponding in number to the PN code length. The PN code usually has a code length of several hundreds to several thousands of bits, as mentioned above. This means that the PN code demodulator requires a considerable scale of hardware and thus is not suited for practical use.
Further, in the conventional code-division multiplex communications, the number of channels that can be multiplexed generally depends upon external noise, and the number of multiplexed channels must be reduced as the code error rate increases. Particularly in an environment in which the quantity of external noise changes with time, it is necessary that the number of multiplexed channels be changed at any point of time. For example, in the course of error-free communications by means of feeble radio waves within a mobile communication band, the communication may suddenly fail due to interference with nearby mobile communications. To cope with such situations, a flexible system is demanded which is able to immediately change the number of multiplexed channels in accordance with the quantity of external noise.
However, the conventional code-division multiplex communication apparatus is unable to detect the quantity of external noise during communications, and accordingly, the number of multiplexed channels cannot be changed immediately in accordance with the quantity of external noise.